new brushless emaxx - awesome!

[Patrick]

If the batteries are getting hot, then you're pulling more current than they can handle. Putting a bigger pinion on will pull more amps and make the problem worse, so if you were using a 16t, don't try an 18t with those batteries. I think the max temp for nimh's was about 55C (130f), definitely not more than 60C.
Also if you keep the same motor and gearing and just add more cells, then with the same driving conditions the higher voltage setup will be able to draw more amps, and heat up the batteries more, like you saw with 12 cells versus 14 cells.
If it was me I would gear down to protect the batteries, or get some better batteries that can handle the current. Since the motor and ESC temps aren't too hot you could keep that gearing with the better batteries and if you get some light ones, like trackpower's there could be room to gear up, because your carrying less weight.